You can certainly create a t-chart and evaluate the function for various x-values. However, to simply sketch the graph, we need some intercepts and an understanding of the graph's behavior via asymptotes.

Firstly, we should understand the function is asymptotic at x = 4, since the
log(0) = inf.

We can also understand that the behavior of the function is opposite of its parent function, log(x), since we effectively have log(-x). Therefore, we should understand that the function decreases from the left, and falls asymptotically at x = 4. Now we just need the intercepts.

The x-intercept is the value of x when y = 0. To figure this out, it's helpful to rearrange the equation in a different form.

y = log2(4 - x) <==> 2y = (4 - x)

20 = 4 - x

1 = 4 - x

x = 3

Now for the y-intercept. Sub in zero for x and solve for y.

2y = 4 - 0

2y = 22

Therefore, y = 2

Plot a vertical dashed line at x = 4, plot your intercepts and sketch the graph, understanding that the value of the function falls from left to right. Develop a t-chart if you want to fill in any other values.